Enhanced Itinerant d⁰ Ferromagnetism and Tunable Topological Properties of a Ca₂C/Ca₂N Heterobilayer

Abstract

Recent studies have shown that a rich variety of exotic physical phenomena can emerge from the presence of electronic flat bands. Using first‐principles calculations, here a monolayered Ca2C possesses a relatively flat band near the Fermi level, and across the whole Brillouin zone is predicted. Next, the weak d0 ferromagnetism in Ca2C is substantially enhanced by a proximity‐coupled electride monolayer of Ca2N is shown. The enhanced magnetism can be attributed to the straightening of the flat band caused by the charge transfer to the electride, while the flattening is tied to the magic ratio of three between the σ and π channels of the effective C─C coupling strength. More surprisingly, even though each of the constituent systems is topologically trivial, the Ca2C/Ca2N heterobilayer is empowered with nontrivial band topology, characterized by a high Chern number tunable by the magnetization orientation or lateral strain. These findings provide a new platform for realizing intriguing magnetic topological metals in 2D materials.